Toolholders of this type, especially those which control numerous machine tools, are known, for example, from the brochure "Computer Numerical Control" of Chiron-Werke GmbH, Applicant's assignee.
In known arrangements of this type, a toolholder with a clamped-in tool is, by means of an arm, removed from a magazine which is disposed either adjacent a spindle head or revolves around the said spindle head. In order to save tool changing time which is necessary in order to remove the arm after clamping-in of the toolholder, and to return it for a subsequent tool change, it is known to dispose the toolholder so that it can be rotated within the arm and so that the arm, during the working of the tool, can remain around the toolholder.
When clamping-in a new toolholder, the toolholder within the arm is introduced from below into the receiving means of the spindle head, while the receiving means of the spindle head rotates at a lower speed, the so-called coupling speed. This is to ensure that a sliding block located at the receiving means locates and engages the matching groove on the toolholder which, at this point, is disposed so as to be rotatable but is halted because corresponding applied forces are lacking.
When the toolholder is moved upward, the sliding block will usually strike the plane countersurface of the toolholder and will slide along on it until it has located and engaged the matching groove. The toolholder will then be pulled into the spindle and will be hydraulically locked there in place.
However, if the friction between the sliding block and the countersurface of the toolholder is too extensive, the sliding block will rest on the countersurface under friction and in this position, will carry the rotatable inside part of the toolholder along in its rotation so that locating and engaging of the sliding block into the groove is delayed or does not take place at all.